This invention relates to both electrochemical and oxygen bleaching or delignification of lignocellulosic materials particularly wood chips and pulp and more particularly to wood pulp prepared by standard pulping methods, especially alkaline pulping methods, and to products prepared thereby and processes for their use.
Chemical pulp is prepared by treating lignocellulosic material with various "pulping chemicals" to render soluble the major portion of the non-carbohydrate portion of the material. The most common chemical pulp is pulp prepared from wood chips by the "kraft" or sulfate process. In this process the wood chips are treated under heat and pressure with sulfide ions in a strongly alkaline aqueous medium. The resulting pulp, while quite strong, is highly colored probably due to a large number of chromophores in the residual lignin. "White" papers are prepared from such pulps and from other chemical pulps by bleaching which principally comprises further delignification. The usual way this is accomplished is by treatment with chlorine-based chemicals such as chlorine, chlorine dioxide, hypochlorite and other oxidative chemicals which oxidize and solubilize the remaining lignin and, thus, remove the chromophoric material.
Recently other oxidative processes employing materials such as oxygen, ozone, peracids and peroxides have been suggested as alternatives to reduce or replace the need for chlorine based chemicals in the bleaching of pulps. For a number of reasons, well known to those in the art, oxygen has proven to be of particular interest and bleaching sequences employing oxygen which are intended to reduce the use of chlorine based chemicals are in commercial operation. However, severe reaction conditions (temperatures greater than 90.degree. and oxygen pressures exceeding 70 psi) are required for standard oxygen-based bleaching sequences as presently practiced.
One convenient means to reduce the severity of oxygen bleaching conditions is to use catalysts which accelerate the reaction between lignin and oxygen. Several such catalysts are known. They are Salcomine (an ethylenediamine-bissalicylaldehyde complex of cobalt), ortho-phenanthroline, and manganese salts. These catalysts are not suitable for practical commercial use because they are relatively expensive due to the fact that they cannot be recovered and regenerated conveniently.
One potential way to generate or regenerate a catalyst for oxygen bleaching is through an electrochemical treatment of the precursor or spent catalyst, respectively.
Electrochemical generation of oxidants or other "electron carriers" in situ or in a closed cycle process in pulp bleaching, and even in some pulping processes for lignocellulosic material, has been experimented with in the past but, as far as is known, with little or no practical success and these processes have never been used commercially.
Electrochemically generated compounds such as hypochlorite, hydrogen peroxide and the like have been shown to react with and solubilize lignin. However, compounds lacking an oxygen function, for example ferricyanide, will react with but not solubilize lignin to any applicable extent unless some oxygen is also present. The prior art has not recognized the importance of the oxygen that was present in providing its reported results and, hence, has not recognized that compounds such as ferricyanide when present in catalytic amounts together with deliberately added quantities of oxygen function as catalysts to solubilize lignin at a very rapid rate under reaction conditions substantially milder than those employed in conventional oxygen bleaching of lignocellulosic pulps. Oxygen bleaching may, therefore, be conducted under milder conditions of temperature and pressure than are presently employed in conventional processes.